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Genome sequence of the bacterium Caulobacter crescentus
By Roberta Friedman

An international research team has sequenced Caulobacter crescentus, an innocuous, single-celled organism that lives in water and shares many genes with pathogens. The microbe, which survives in nutrient-poor habitats, has one circular chromosome containing some 3,700 genes. But this simplicity is deceptive: A single C. crescentus cell divides into two different types of cells, swarmer cells and stalked cells. Thus, the genome sequence may prove useful for investigating aspects of developmental biology such as the mechanisms of asymmetric cell division.

Detail from image showing two types of C. crescentus cells: flagellated swarmer cells and stalked cells. View larger

"C. crescentus is a simple and highly manipulable single-celled model system to study cellular differentiation, asymmetric division, and their coordination with cell cycle progression," the researchers write in Proceedings of the National Academy of Sciences. "Caulobacter does all that with less than 4,000 genes." William C. Nierman, of The Institute for Genomic Research (TIGR) in Rockville, Maryland, led the sequencing of the genome's four million units of DNA, or base pairs.

Like stem cells, the stalked cell continually gives rise to a new swarmer cell at each cell division. The stalk is a thin cylinder growing out of one pole of the cell, which is used to adhere to rocks in fast-flowing streams. The motile swarmer cell has a flagellum. "Caulobacter is an excellent model system to study one of the most important questions in developmental biology: What is the mechanism of an asymmetric cell division?" says Lucy Shapiro, a member of the team and Director of the Beckman Center, Stanford University School of Medicine in California.

The researchers compared C. crescentus to other completely sequenced organisms such as E. coli and Xylella fastidiosa. Comparative analyses may yield clues to the pathogenicity of infectious agents, such as the Rickettsia agent that causes typhus. Rickettsia prowazekii has adapted to life inside human cells but has lost many genes required for independent survival.

The global comparison between the C. crescentus genome and the genome of Rickettsia "may ultimately help to discriminate between the 'core' cell cycle genes required for proper progression of the cell cycle and the 'peripheral' genes that are cell-cycle regulated but not needed for full viability in C. crescentus," the researcher write. The R. prowazekii genome was sequenced in 1998 by a group at the University of Uppsala, Sweden.

Genes known to be crucial to the cell cycle in the C. crescentus genome, such as ctrA, parAB, and recA, have counterparts in the genome of R. prowazekii. Other Caulobacter genes, such as those coding for its flagellum, do not have counterparts in the Rickettsia genome, the researchers determined.

Analysis of the Caulobacter genome reveals surprising similarities with soil-dwelling microbes. Even more startling to the researchers is the presence of genes coding for enzymes that break down plant polysaccharides, such as cellulose, lignin and pectin, as well as transporter systems for importing the resulting sugars. Plants therefore serve as a previously unrecognized source of fuel for Caulobacter.

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Nierman, W.C. et al. Complete genome sequence of Caulobacter crescentus. Proc Natl Acad Sci USA 98, 4136-4141 (March 27, 2001).

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